Generally, in a wireless receiver, a high-frequency radio signal from an antenna is firstly converted into a signal with an intermediate frequency (IF (Intermediate Frequency) signal), and then the IF signal is amplified and inputted to a detector. In the detector, a desired signal process, namely, a detection process in which information included in the radio signal is taken out is performed. In this detection process, the gains of the radio signal and the IF signal are set according to a signal strength of the electric wave received by the wireless receiver, so as to be in a range in which the detection process can be desirably performed.
The signal strength of an electric wave is generally referred to as a received signal strength indicator (RSSI), and measured by an RSSI circuit system.
In portable phones including an IQ modulator/demodulator and the like, an example of the RSSI is a direct current voltage generated by a diode performing envelope detection of a received signal whose band is limited by an IF filter. The RSSI is also used for generating various control signals by being inputted to a base band process circuit.
As illustrated in FIG. 14, an RSSI circuit system using a conventional IF amplifier circuit is primarily constituted of an IF amplifier current source circuit 1210, an IF amplifier circuit 1220, an RSSI amplifier circuit 1230, and a conversion circuit 1240 (see Patent Document 1). These circuits are often integrated in, for example, an LSI (integrated circuit).
The IF amplifier current source circuit 1210 supplies a current to the IF amplifier circuit 1220. The IF amplifier circuit 1220 is made by serially cascading amplifiers 1221, 1222, 1223 and 1224, and amplifies an input signal supplied from, for example, an exterior signal source 1270, and outputs the signal from each stage. The RSSI amplifier circuit 1230 converts voltages outputted from the respective stages of the amplifiers 1221, 1222, 1223 and 1224 in the IF amplifier circuit 1220, and adds up the currents of the respective stages to output an RSSI signal current.
The conversion circuit 1240 converts the RSSI signal current, based on conversion characteristics set by a reference resistance 1250, so as to correct influence of temperature dependence to an extent practically negligible. An output current converted by the conversion circuit 1240 is outputted from an output terminal 1260 to, for example, an external signal strength display device.
Each of the amplifiers 1221, 1222, 1223 and 1224 in the IF amplifier circuit 1220 is, for example, a differential amplifier circuit. To be specific, as illustrated in FIG. 15, the differential amplifier circuit includes load resistors 1321 and 1322, transistors 1323 and 1324, input terminals 1325 and 1326, an output terminal 1327, and a current source 1328. The transistors 1323 and 1324 constitute a differential amplifier. Based on a current supplied from the IF amplifier current source circuit 1210, the current source 1328 outputs a current, which essentially becomes an amplification current source for the transistors 1323 and 1324.
(Patent Document 1) Japanese Laid-Open Patent Application No. 46341/2003 (published date: Feb. 14, 2003)
The current supplied from the IF amplifier current source circuit 1210 varies depending on voltage Vt and inner resistance R. As such, an output from the output terminal 1327 also varies depending on voltage Vt and inner resistance R.
However, when the input level is low and accordingly the amplifiers 1221, 1222, 1223 and 1224 of the respective stages of the IF amplifier circuit 1220 do not saturate, outputs from these amplifiers do not have temperature dependence.
On the other hand, there is a case where the input level is high and accordingly the amplifiers 1221, 1222, 1223 and 1224 of the respective stages of the IF amplifier circuit 1220 saturate sequentially from a later stage to a previous stage. In such a case, the output from each of the amplifiers 1221, 1222, 1223 and 1224 becomes the product of the load resistors 1321 and 1322 and a current from the current source 1328, and shows high (strong) temperature dependence directly proportional to Vt (Vt=kT/q; here, k is a Boltzmann constant, T is absolute temperature, and q is an amount of electrical charge).
As a result, the output of the RSSI amplifier circuit 1230 also varies depending on a change in Vt. The change causes an error in the output from the RSSI circuit system (a result of measurement of received signal strength).
Therefore, in the conventional RSSI circuit system, the output of the RSSI amplifier circuit 1230, which varies depending on Vt, is converted by the conversion circuit 1240 based on conversion characteristics set by the reference resistance 1250, so that the influence of temperature dependence is corrected to an extent practically negligible.
As described above, in the conventional RSSI circuit system, there is a problem that temperature dependence of an output amplitude (gain) varies greatly depending on the input level to the IF amplifier circuit 1220, with the result that output characteristics vary depending on temperature and therefore may cause error. When the input level is low, the amplifiers 1221, 1222, 1223 and 1224 of the respective stages in the IF amplifier circuit 1220 do not saturate. When the input level is high, the amplifiers 1221, 1222, 1223 and 1224 of the respective stages in the IF amplifier circuit 1220 saturate, or almost saturate.
Further, in the conventional RSSI circuit system, there are such problems that in order to solve the problem of temperature dependence of an output, the conversion circuit 1240 is needed and accordingly the whole circuit arrangement becomes complicated. Another problem is that, when the RSSI circuit system is integrated in LSI, an additional space for integrating the conversion circuit 1240 is needed, which makes it difficult to downsize or integrate the circuit. Further, power consumption is increased due to the conversion circuit 1240.